right of citizens to vote shall not be denied or abridged by US or by any State on account of race, color, or previous condition of servitude

Amendment 19

right of citizens of US to vote shall not be denied or abridged by US or by any State on account of sex

Amendment 24

right of citizens to vote in any primary or other election for President or Vice, or for Senator or Representative in Congress, shall not be denied by reason of failure to pay any poll tax or other tax

What factors give incumbents in House and Senate the edge in election campaigns?